The present invention relates to a three-port solenoid-operated valve having an inlet port, an outlet port, and a relief port.
Three-port solenoid-operated valves have found wide use as directional control valves which interconnect a pipe for delivering a fluid under pressure to a hydraulically operated device, a relief pipe for bleeding off the fluid under pressure from the hydraulically operated device, and a supply pipe extending from a hydraulic pressure source. One typical type of three-port solenoid-operated valve has a fluid passage defined in a cylindrical solenoid and having ports at its ends, and a plunger disposed in the fluid passage and movable in response to energization of the solenoid. The plunger has valve bodies disposed on its ends and movable into and out of contact with the respective valve seats in the ports. The valve bodies are biased by respective springs for assisting themselves in opening and closing the ports.
Since the fluid in the hydraulic circuit is normally kept under high pressure, the springs used in the three-port solenoid-operated valve for biasing the valve bodies are constructed to produce large spring forces. If these springs fail to function properly due to fatigue, then the valve will not operate to open and close the ports as desired. When the spring biasing the valve body to open the inlet port, in particular, fails to function, then the inlet port is not opened since the spring that urges the same valve body to close the inlet port is strong enough to keep the inlet port closed. Therefore, the three-port solenoid-operated valve does not operate even if there is a hyraulic pressure applied to the inlet port by the hydraulic pressure source.